Liquid crystal polymers are a family of materials that exhibit a highly ordered structure in the melt, solution, and solid states. They can be broadly classified into two types; lyotropic, having liquid crystal properties in the solution state, and thermotropic, having liquid crystal properities in the melted state.
Further discussion of liquid crystal polymers hereinbelow refers only to thermotropic liquid crystal polymers, i.e., those liquid crystal polymers and liquid crystal polymer alloys which are processed in the melted state. Also, liquid crystal polymer, as used herein, is meant to include polymer alloys having a liquid crystal polymer component as well as liquid crystal polymers alone. For convenience, the term "liquid crystal polymer" is used herein to include material of both kinds.
Most liquid crystal polymers exhibit excellent physical properties such as high strength, good heat resistance, low coefficient of thermal expansion, good electrical insulation characteristics, low moisture absorption, and are good barriers to gas flow. Such properties make them useful in a broad range of applications in the form of fibers, injection molded articles, and, in sheet form, as electronic materials for printed circuit boards, packaging, and the like.
Many of the physical properties of liquid crystal polymers are very sensitive to the direction of orientation of the liquid crystal regions in the polymer. The ordered structure of the liquid crystal polymer is easily oriented by shear forces occurring during processing and highly aligned liquid crystal chains can be developed that are retained in the solid state, and result in highly anisotropic properties. This can be highly desirable for certain products, for example, in filaments, fibers, yarns, and the like. Anisotropic properties are often not desirable, however, in products having planar forms, such as tape, films, sheet, and the like.
A number of methods are used to produce liquid crystal polymer materials in planar forms that have more balanced, less anisotropic properties. These include the use of multilayer flat extrusion dies which are fashioned such that they extrude overlapping layers at intersecting angles, use of static mixer-agitators at the die inlets, and the like. More recently, dies having rotating or counter-rotating surfaces have become known in the art and successfully used. These extrusion techniques, used separately or in combination with other methods known in the art, such as film blowing, can produce liquid crystal polymer film and sheet that are multiaxially oriented, that is, oriented in more than one direction, and have more balanced physical properties.
Also known in the art are methods to produce planar forms in which a liquid crystal polymer film is laminated to a porous polymeric support membrane to form a composite sheet, and the composite sheet then stretched in one or more directions to multiaxially orient the liquid crystal polymer. Such methods are disclosed in European Patent Application No. EP 0 612 610 and U.S. Pat. No. 5,534,209 (to Moriya).
Unlike most other thermoplastic polymer resins, thermotropic liquid crystal polymers form high-viscosity melts having thixotropic characteristics. The melt viscosity of such materials, and the orientation of the liquid crystal polymer domains, are substantially altered in response to shear forces applied to the melted material. As noted above, these attributes can be very useful and are taken advantage of in the manufacture of articles having anisotropic properties and, to a large degree, can be controlled in the manufacture of articles having planar forms such as sheet, film, or other flat-shaped objects in which more balanced properties may be desired. However, due to these attributes, it is extremely difficult to form hollow or other complex-shaped articles of liquid crystal polymers using molding techniques commonly used with many other thermoplastic polymer resins, especially techniques such as blow-molding or vacuum-forming, in which a gas pressure differential across a polymer preform is used to force the preform against the molding surfaces. Consequently, molded articles of liquid crystal polymers formed by such techniques have yet to be developed.
It is a purpose of this invention to provide complex-shaped articles comprising a liquid crystal polymer, and further to provide methods by which such articles can be made by gas-pressure-differential molding techniques.